1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Background Description
Liquid crystals recently have gained considerable importance primarily as dielectrics in indicating, devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical indicating devices which utilize liquid crystal cells are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, deformation of aligned phases (DAP type), the Schadt-Helfrich effect (rotation cell), the "guest-host effect" or a cholestericnematic type phase transition.
Liquid crystals must satisfy a number of requirements in order to be suitable as dielectrics for electro-optical indicating devices. For example, the liquid crystals must have a high chemical stability towards environmental factors (e.g. heat, air, moisture and the like), must be photochemically stable and colorless, must have short response times, must not be too high a viscosity, must have a nematic or cholesteric-type mesophase in all temperature ranges in which the liquid crystal cell is to be operated, and must give a good contrast. Other properties such as, for example, the threshold potential, the dielectric anisotropy and the electrical conductivity must fulfill different conditions depending on the type of cell which is used.
Since, in general, it is not possible to achieve all desired and to some extent contradictory properties with a single compound, attempts have been made to use several components so as to produce optimal properties for the particular application. In this case it is, however, important that the components undergo no chemical reactions with one another and can be mixed well. Further, ideally the mixtures should have no or small smectic mesophases.
Moreover, the customary liquid crystal mixture components with low viscosities generally have only low clearing points or are not liquid crystalline. Thus, components having high clearing points must frequently be added to the mixtures. Disadvantageously, the latter components generally greatly increase the viscosity of the mixtures.
Furthermore, liquid crystals have also been effectively used as stationary phases in gas chromatography. However, the known stationary phases have, above all, the disadvantage that they can be used only up to temperatures of about 180.degree. C.
Through the present invention, novel liquid crystalline compounds have been found which avoid the aforementioned disadvantages.